This invention relates to improvements in an MOS transistor.
Conventional N channel MOS transistors have comprised an N.sup.+ type source region and the N.sup.+ type drain region disposed in spaced relationship on one of the opposite main faces of a P.sup.- type silicon substrate and a gate electrode formed of a polycrystalline silicon layer disposed on a silicon dioxide film subsequently disposed on the one main face of the substrate between the source and drain regions. When the distance between those source and drain regions, i.e. the channel length becomes short, the application of a voltage across the two regions has resulted in the punch-through of the transistors which does not greatly affect the potential on the polycrystalline silicon layer.
In order to avoid the punch-through, it has been already proposed to increase the impurity concentration of the silicon substrate or to decrease the depth of the source and drain regions. The former method has brought about an increase in the rate of change in the threshold voltage or a body effect constant due to a back gate voltage applied across the source region and the silicon substrate. The latter method has resulted in the disadvantage that the breakdown voltage of the PN junction decreases because the PN junction has a small radius of curvature.
Therefore conventional N channel MOS transistors cannot be used to form integrated circuits having a high scale integration. This is also true in the case of conventional P channel MOS transistors.
Accordingly it is a general object of the present invention to improve MOS transistors.
It is an object of the present invention to provide a new and improved MOS transistor normally operative even though it has a short channel length yet which is not subject to punch-through.
It is another object of the present invention to provide a new and improved MOS transistor suitable for forming large scale integrated circuits.
It is still another object of the present invention to provide a new and improved MOS transistor which does not have an increased threshold voltage, an increased body effect constant or a decreased breakdown voltage of the PN junction.
It is a still further object of the present invention to provide a new and improved MOS transistor having a high drain breakdown voltage.